A typical hydraulic power transmission system includes a pump to supply high pressure hydraulic fluid to a hydraulic valve. The valve controls the fluid pressure of the working hydraulic fluid supplied to a hydraulic actuator. The hydraulic actuator then develops an output force proportional to the pressure of the working fluid supplied thereto. These components form the high pressure side of the system. The output force of the actuator is useful to act on a load. Accordingly, the resultant force which acts on the load is determined by the valve orifice cross section area. The hydraulic fluid returns to a reservoir on the low pressure side of the system from which it may again be pumped.
The hydraulic actuator may provide the sole force acting on the load or may provide an assist force in addition to a mechanical force acting on the load. In this latter class of hydraulic power transmissions systems, commonly known as a hydraulic servo assist system, the force developed by the hydraulic actuator is developed commensurately with the sensed mechanical force acting on the load to provide a force or assist gain.
In the parent application hereto, U.S. Ser. No. 07/738,193, now U.S. Pat. No. 5,135,070, the entirety of which is incorporated herein by reference, a system for the active control of the pressure of the working fluid in the high pressure side of the hydraulic power transmission system is disclosed. The disclosed system is operable to control both the static pressure of the working fluid, so that a predetermined hydraulic force actuation profile or an assist gain profile is realized, and the dynamic pressure of the working fluid so that pressure fluctuations caused by external or internal influences are cancelled in real time.
It is to be understood and it will become apparent that the invention disclosed hereinbelow is also operable with any type of hydraulic power transmission system. To enable one skilled in the art to more fully appreciate the breadth and scope of the present invention, the unique aspects and limitations of the hydraulic servo assist system are set forth below to demonstrate the adaptability of the present invention to various types of hydraulic systems.
In the hydraulic servo assist system, a pump draws hydraulic fluid from a reservoir and pumps such fluid through a high pressure supply line to a hydraulic servo assist valve. Generally, as the mechanical apparatus acts on the load, the servo assist valve senses or detects the mechanical force presently acting on the load. The servo valve is actuated in response to sensed mechanical force with the magnitude of actuation, which determines a servo valve orifice cross sectional area, being commensurate with the mechanical force. As the servo assist valve is actuated, fluid pressure is allowed to build in a hydraulic actuator coupled to the load. The high pressure fluid in the hydraulic actuator provides a hydraulic force which is additive to the mechanical force acting on the load.
The pressure of the hydraulic fluid in the actuator, and hence the amount of hydraulic force provided, is determined by the quiescent or static pressure of the high pressure side of the system and the increase of pressure occurring as a result of servo valve actuation changing the servo assist valve orifice cross sectional area. On the low pressure side of the system, the fluid is returned from the servo assist valve and actuator to the reservoir through a low pressure return line.
Therefore, it is readily apparent that the hydraulic servo assist system provides a force gain to the mechanical actuation of the load. The servo assist gain is seen from the above to be a function of both the pressure of the hydraulic fluid in the high pressure supply line and the cross sectional area of the servo assist valve orifice, since these system variables determine the pressure of the hydraulic fluid acting on the load through the hydraulic actuator.
A motor vehicle power steering system is one particular example of the hydraulic servo assist system as described above. Actuation of the steering tires of the vehicle, which is the load referred to above, is provided primarily through a steering gear which couples driver input at the steering wheel to the tires with the hydraulic servo assist system providing the assist force to the steering gear while mechanical force is being imparted to the steering gear through the steering column. Although the construction of a motor vehicle servo assist power steering system is well know, the parameters and constraints on such system imposed by its operating environment have resulted in an elegant high performance system with a unique set of features and limitations, especially in pump and servo assist valve design. Accordingly, the general nature of these components is described for the convenience of the casual reader hereof.
Usually, the steering wheel is connected to the steering gear through a steering column and the steering gear is in turn connected to tie rods, each of which interconnects a respective one of the steerable spindle/hub assemblies of the vehicle suspension system to the steering gear. The spindle/hub assemblies are mounted for rotation through a prescribed arc about a king pin axis, as is well known. The steering tires are mounted to their respective spindle/hub assembly so that the rotation of the spindle/hub steers the tires. The function of the steering gear is to convert the rotation of the steering column to linear motion of the tie rods.
The rack and pinion type steering gear is presently in the most common use because of its simplicity, compact dimensions and directness of action. In this type of steering gear, the pinion meshes with an elongated rack so that rotation of the pinion is translated to linear displacement of the rack. In its simplest form, the pinion is carried at one end of the steering column and the tie rods are attached to the rack.
Other types of motor vehicle steering gears include the recirculating ball steering gear and the worm and roller steering gear. Any of these various types of steering gears may be provided with hydraulic servo assist to reduce steering effort required at the steering wheel to turn the steering tires during vehicle maneuvers. The design of the hydraulic servo assist system, and in particular the servo assist valve, may depend on the type of steering gear utilized.
The discussion hereinbelow will, for the sake of brevity and convenience, refer only to the rack and pinion type steering gear and the particular design constraints of the servo assist system imposed by such steering gear, since these designs are most commonly used. However, this discussion is not intended to limit the utility or scope of the present invention.
In the servo assist power steering system, the hydraulic pump is usually a conventional vane type pump which is belt driven from the engine crankshaft. The volume of hydraulic fluid moved by the vanes therefore increases with increasing engine speed. The engine speed dependence on the volume of fluid moved through the vanes would normally cause fluid pressure in the high pressure side of the hydraulic system also to be dependent on engine speed. However, the power steering assist system requires a generally stable steady state or static pressure in the high pressure side of the system over an indeterminately varying operating speed of the pump vanes. Therefore, a constant volume output flow is required by this system.
The constant output flow in turn requires that hydraulic fluid be continuously recirculated through both the high pressure and low pressure sides of the servo assist system, wherein the pressure drop occurs across the servo assist valve orifice, even when no useful work is being performed by the system. A restriction in the flow path, such as a decrease in the cross sectional area of the servo assist valve orifice without the corresponding actuation of the steering rack, would then cause excessive high pressure at the pump output. The typical pump used in servo assisted steering therefore includes both output flow regulation and output pressure regulation. Both of these functions are accomplished by flow control and pressure relief valves at the pump output.
The flow control valve is spring biased and is acted upon by the pressure of the fluid entering a venturi tube in opposition of the spring bias and also by the pressure of the fluid exiting the venturi tube in support of the spring bias. The venturi tube is located at the pump vane output to receive fluid. According to well known principles, the pressure of fluid exiting the venturi tube decreases as its flow increases. The net force acting on the flow control valve actuates this valve to return excessive flow from the output of the pump vanes prior to the fluid entering the venturi tube to the low pressure side of the system at either the pump input or reservoir.
The pressure relief valve is a spring biased check ball which unseats in response to excessive pressure of the fluid exiting the venturi tube. When the pressure relief valve opens, fluid exiting the venturi tube is returned to the low pressure side of the system, thereby reducing flow at the pump output and hence pressure. Both valves may also act in concert so that a relatively stable output flow is maintained.
The steering gear servo assist valve has three primary components, which are a first valve member, a second valve member, and a torsion bar. The torsion bar interconnects the first and second valve members and further provides an axis of relative angular displacement between each valve member. The first and second valve members are coaxially disposed with one valve member being radially disposed within the other valve member. One valve member is connected to the steering column and the second valve member, in turn, is connected to the pinion. The torsion bar thus provides the sole mechanical connection between the steering column and the pinion.
Normally, the first and second valve members are at their steady state position relative to each other which is defined as the position in absence of any torsional force in the torsion bar. In the steady state position, the fluid flow is continuous through the valve orifice between the high and low pressure sides of the system. The valve orifice is defined by an input opening in the first valve member and into a passage between the valve members. The passage diverts into two branches with each branch exiting the second valve member through a respective output opening. The servo assist valve also includes two channels, each of which communicates one branch of the valve orifice passage with a respective chamber on either side of a double acting piston, which functions as the hydraulic actuator. In the steady state position, each channel is equally open to the valve orifice passage. Fluid is introduced into or removed from either chamber only through the channel in communication therewith. The piston is connected to the steering rack.
Steering inputs at the steering wheel will, because of the torsion bar, cause relative angular displacement to occur between the first and second valve members. This relative angular displacement will cause one branch of the passageway to become restricted at its termination at its respective output opening in the second valve member and more open with respect to the input opening in the first valve member. Conversely, the other branch of the passageway becomes restricted to flow from the input opening in the first valve member and more open at its termination with its respective output opening in the second valve member. The effective cross sectional area of the servo assist valve orifice thus decreases during increasing relative angular displacement allowing pressure to build in the branch of the passageway restricted at its termination with the output opening of the assist valve while the pressure of the other branch decreases as it becomes more open to the output opening in greater communication to the low pressure side of the system. Accordingly, fluid pressure builds in the high pressure side of the system.
This pressure imbalance in each branch of the passageway is transferred to the chambers on each side of the actuator piston through the channels which communicate one branch with its respective chamber. As the piston moves from the high pressure chamber, the high pressure chamber expands receiving high pressure fluid from the channel in communication with the high pressure passageway and the low pressure chamber contracts pushing its fluid through the channel into the low pressure branch of the passageway. The piston thus converts the pressure differential in each branch of the passageway into a force acting on the steering rack.
The primary force acting on the steering rack is developed by steering inputs at the steering wheel and coupled through the torsion bar and pinion. As the servo assist valve members displace as described above, the force developed by the hydraulic actuator assist the mechanical force. This assist force then reduces the torsional force acting on the torsion bar thereby returning the servo assist valve to its steady state position.
The servo assist gain is primarily determined by the torsional stiffness of the torsion bar. As the torsion bar is made less stiff, the magnitude of the relative angular displacement of the servo valve members is greater at the initiation of a steering input than it would be if the bar is made torsionally stiffer. As described hereinabove, for increasing magnitude of relative angular displacement between the valve members, there is a corresponding increase of assist pressure developed in the branch of the valve passageway which is becoming more restricted at its termination. Accordingly, the hydraulic servo assist acting on the steering rack increases as the stiffness of the torsion bar decreases and vice versa.
The amount of servo assist, or the servo force gain, is selected to maintain "road feel" at the steering wheel. Usually, excessive assist or high gain diminishes road feel and insufficient assist or low gain causes unacceptably high steering effort. In a typical motor vehicle, required steering effort is at a maximum when the vehicle is at rest, and decreases with increasing vehicle speed. This effect is due to the decreasing rolling and scrub resistance of the tires as vehicle speed increases.
The above described servo assist system is useful for decreasing the amount of steering effort required when the vehicle is at rest or at very low vehicle speeds. However, as vehicle speed increases, the servo gain developed by the above system acting on the steering rack remains constant. This may result in excessive assist provided by the system at higher vehicle speeds, thereby degrading road feel at these speeds. However, road feel becomes even more important at higher speeds where the degree of steering control necessary for the operator of the vehicle to respond to various road situations, especially those which necessitate abrupt evasive maneuvers, increases. To maintain road feel at higher speeds, several types of prior art devices have been developed which function to decrease servo gain of the system as the speed of the vehicle increase.
A first prior art device, commonly referred to as an electronic variable orifice (EVO) system, modifies the flow rate of hydraulic fluid discharged from the engine driven hydraulic pump. The EVO system includes a restriction to the flow exiting the venturi tube. The restriction may be in the form of a pin which is disposed external to and coaxial with the exit orifice of the venturi tube. As vehicle speed increases, the pin is gradually inserted into the exit orifice thereby decreasing the effective cross sectional area of this orifice. The pin is actuated by an electronic speed sensing actuator.
The effect of the pin being inserted into the exit orifice of the venturi tube is to further decrease the pressure of the flow exiting the venturi tube, thereby causing increased actuation of the flow control valve. As the flow control valve, due to the presence of the pin, diverts additional flow from the output of the pump vanes prior to entering the venturi tube back to the low pressure side of the system, as described above, the flow exiting the pump and supplied to the servo assist valve accordingly is reduced. The reduced flow, when the servo valve members are displaced, will result in a reduced pressure buildup in the restricted branch of the valve orifice passageway. The reduced pressure buildup thus reduces the servo assist force developed by the hydraulic actuator, and hence the servo gain of the system.
A disadvantage and limitation of the the EVO system is that assist gain is variable over a limited range of assist pressures. For example, at low assist pressure the output flow of the pump may become insufficient to effectively build pressure in the restricted branch of servo assist valve passageway, thereby not allowing sufficient servo force to act on the steering rack to return the servo valve to its steady state position. Another disadvantage and limitation is that the EVO system at reduced flow rates may not respond effectively to sudden transient maneuvers occurring in rapid succession, such as a swerve from and a recovery to a traffic lane to avoid an object. This lack of response is again due to the reduced flow through the servo valve wherein assist pressure may not build quickly enough to provide initial assist to the maneuver. The effect is that the driver senses a delay in assist or a pulsing of the steering wheel.
A second prior art device utilizes a reaction chamber in conjunction with the servo assist valve. The reaction chamber receives increasing amount of hydraulic fluid as vehicle speed increases. The fluid in the reaction chamber acts against a spring bias. As the spring bias is overcome, the reaction chamber actuates a mechanical gripping or clutching apparatus which is operative to limit increasingly with increasing vehicle speeds the total magnitude of relative angular displacement between the first valve member and the second valve member of the servo assist valve. Such mechanical action may be typically achieved by V-shaped detents which receive steel balls which are urged into the detents by action of the reaction chamber. Eventually, the first valve member and the second valve member become completed locked together so that servo assist is completely removed from the system.
The second prior art device does not exhibit the same problems of the above described EVO system. However, a disadvantage and limitation of the second type of prior art device is that it significantly increases the mechanical complexity of the servo assist valve and the number of precision manufacturing steps which need be performed to manufacture the reaction chamber and clutch actuator. In the highly cost competitive automobile industry, this type of device may only find limited usefulness in certain high end vehicles. The EVO system may be more suitable to the broad price range spectrum of vehicles.
Yet a third type of prior art system also utilizes a reaction chamber which receives hydraulic fluid in response to increasing vehicle speeds. However, instead of restricting the relative angular displacement of the servo valve, the reaction chamber is used to introduce a resistive or frictional force on the steering column itself so that the operator of the motor vehicle senses greater steering effort.
The third type of prior art device is very simple and of low cost. However, a significant disadvantage and limitation of the this device is that the introduction of a resistive force on the steering column may result in an artificial steering feel as sensed by the driver and a lack of refinement in the steering system. Accordingly, this device may find limited usefulness only at the extreme low end of the vehicle cost spectrum.